1. Field of the Invention
The present invention relates to an information recording disc demagnetization apparatus for demagnetizing information recording discs whose information is read by irradiating a laser beam, such as a music CD (audio compact disc), a photographic CD, a game software CD, a DVD (digital video disc), an MD (mini disc) or a CD-ROM (read only memory), for the purpose of recovering tone quality, image quality and the like deteriorated by magnetism during reproduction.
2. Description of the Related Art
An information recording disc such as a music CD (audio compact disc), a photograph CD, a game software CD, a DVD (digital video disc), an MD (mini disc) or a CD-ROM (read only memory), in which various data and software is recorded, are made of resin such as polycarbonate and amorphous polyolefin, having a recording reflecting surface for recording a signal made of an aluminum foil. It is a widely accepted theory that information recording discs made of resin and aluminum foil are non-magnetic and, therefore, are not influenced by magnetism.
It is also generally believed that the information recording disc is a digital recording medium whose recorded information does not deteriorate even if it is reproduced a number of times. However, it has been found that tone quality and image quality deteriorate when the information recording disc is reproduced several times, and that after demagnetization processing is conducted on an information recording disc, the tone quality and image quality is improved when compared to the tone quality and image quality prior to the demagnetization processing.
It is therefore believed that the information recording disc, which is normally not considered to be influenced by magnetism is, in fact, magnetized during the reproduction process. To confirm this theory, the inventor of the present invention carried on experiments upon a music CD that had been used for 1.5 years and on a music CD that had been used for 2 years (both of which had been reproduced several tens of times). The quantity of magnetism of the label and signal surfaces thereof were measured with a gauss meter.
According to the results of the above experiments, the quantity of magnetism on the printed portion of a CD is a maximum of 1.5 gauss, considerably larger than the 0.5 gauss measured on other portions of the CD of magnetism. In fact, if a CD is suspended by a string, and a magnet is brought near the CD, the CD is slightly attracted to the magnet. Therefore, it is obvious that the information recording disc will be somewhat affected by magnetism.
It is believed that this increased magnetism found in the printed portion of an information recording disc such as a CD is due to magnetic substances contained in the paints with which the title of the CD, titles of musical compositions, names of performers, the name of a manufacturer, and the like, are printed on the rear surface (label surface) of the information recording medium.
The magnetic substances present in these paints are, for example, iron oxide, which is found in red paints, and cobalt, which is found in blue paints. In addition, aluminum foil, which is present on the recording reflecting surface of the information recording disc, is not 100 percent aluminum but, rather, contains small amounts of magnetic substances such as nickel, cobalt and iron. It is hypothesized that such magnetic substances are magnetized by the magnet of the pick-up unit of a reproduction apparatus.
Due to the fact that laser beams at the pick-up unit of the reproduction apparatus are electromagnetic waves, they are naturally deflected, refracted, distorted or the like by the influence of the magnetism of the magnetic information recording disc. As a result, tone quality and image quality deteriorate when the information recording disc is reproduced. It is therefore desirable to sometimes conduct demagnetization processing after the disc has been reproduced several times.
Demagnetizers for reducing or erasing the magnetism of a target apparatus have previously been available. Such demagnetizers include, for example, a tape deck head eraser as shown in FIG. 14, a demagnetizer for magnetic tapes as shown in FIG. 15, a demagnetizer for demagnetizing a machine tool such as a drill shown in FIG. 16 and an apparatus for improving the tone quality and image quality of a CD as shown in FIG. 17, which can overcome the problem of the magnetization of an information recording disc which is a target of the present invention.
The demagnetizers shown in FIGS. 14 and 15 can demagnetize an information recording disc (such as a CD) by tracing the upper surface thereof, whereas the demagnetizer shown in FIG. 16 can demagnetize an information recording disc by moving the disc thereon (an example of diversion).
In the CD tone quality and image quality improving apparatus shown in FIG. 17, the hole in the central portion of a CD is mounted onto a protrusion on the central portion of the demagnetizer, a start switch is depressed to rotate the CD at a high speed of several tens of seconds within the entire magnetic field which consists of the strong magnetic field of a permanent magnet, plus the magnetic field of a coil. This type of apparatus is described in U.S. Pat. No. 5,487,057 (patented on Jan. 23, 1996) in detail. However, if a magnetized information recording disc is rotated at high speed, as in the prior art apparatus mentioned above, it functions as a kind of dynamo thereby shaking its surroundings and inducing an electromotive force to surrounding wires, the power supply, and causing jitters.
The purpose of the CD tone quality and image quality improving apparatus 31, as shown in FIG. 18, is to reduce the relaxation noise of an information recording medium. The apparatus consists of a means for creating a modulated electromagnetic field, and a means for rotating the information recording medium to pass it through the modulated electromagnetic field. As shown in FIGS. 18 and 19, the apparatus includes a magnetic unit 32 consisting of permanent magnets 22 and a coil 23 wound around the magnets 22. By rotating the information recording medium or CD 25 at high speed within a magnetic field (the strong magnetic field of the permanent magnets 24)+(the magnetic field of the coil 23) created by the magnetic unit 32 for several tens of seconds by a motor 35, relaxation noise, that is, lingering noise within the recording medium conductor (or aluminum foil) is attenuated. The apparatus 31 further comprises a control switch 27, and a DC power supply 29.
The tape deck magnetizer shown in FIG. 14 is designed for a cassette tape and demagnetizes a narrow area. To demagnetize the entire surface of the information recording disc, it is necessary to move the demagnetizer over the recording medium a number of times. Therefore, use of this demagnetizer is time consuming and uniform magnetization is difficult to attain.
The magnetic tape demagnetizer shown in FIG. 15 is slightly wider than the tape deck magnetic eraser shown in FIG. 14. However, disadvantages in operability, time and uniform demagnetization remain. The tool demagnetizer shown in FIG. 16 is large and heavy. Therefore, it is not useful in demagnetizing home information recording discs.
In contrast to the apparatuses shown in FIGS. 14-16, the CD tone quality and image quality improving apparatus shown in FIGS. 17-19 is more convenient, time efficient and easier to operate. However, the apparatus fails to satisfactorily demagnetizate an information recording medium.
The apparatus shown in FIGS. 17-19 is intended to enhance the uniform magnetization direction of a magnetic substance by forcedly applying stronger magnetism to the substance rather than to carrying out demagnetization. Therefore, even if the effect of improving the tone quality and image quality is provided, the magnetic substances contained in the CD are uniformly magnetized and uniformly distorted. Laser beams from the pick-up of the hardware are also uniformly deflected. It is believed that, as a result of this uniform deflection, a deterioration in tone quality and image quality appears.
It is quite obvious that, by exposing the CD to the magnetic field of permanent magnets, magnetization of magnetic substances present in extremely small quantities in the CD occurs. The prior art apparatus described above does not, therefore, serve as a "demagnetizer".
As described above, the CD tone quality and image quality improving apparatus is advantageous in that the demagnetization process can be conducted on the entire surface of the CD at once. However, since this system utilizes high-speed rotation of the CD through the magnetic field of permanent magnets, magnetism may be increased or a non-magnetic CD may be magnetized depending on the state of magnetism on the CDs. It is also feared that a kind of dynamo phenomenon may occur due to the high speed rotation of the CD within the magnetic field. It should also be pointed out that a user might misunderstand that the CD has been effectively demagnetized because the tone quality or image quality changes after use of the demagnetizer.